1. Field of the Invention
The present invention relates to an air-fuel ratio sensor for use in an atmosphere of unburnt gas.
2. Description of the Related Art
Air-fuel ratio sensors for detecting an air-fuel ratio in an atmosphere of unburnt gas from the amount of oxygen after reaction of fuel vapor with oxygen are known heretofore. One such known air-fuel ratio sensor includes, as shown here in FIG. 25, a cylindrical housing 10′ and an air-fuel ratio sensor element 3′ inserted through the housing 10′ and capable of detecting an air-fuel ratio of an unburnt gas atmosphere.
The known air-fuel ratio sensor 9 also has an atmospheric side cover 12′ disposed on a rear end of the housing 10′ and defining an inside chamber for storing atmospheric air, and a measured gas side cover 2′ disposed on a front end of the housing 10′ so as to cover the air-fuel ratio sensor element 3′ and defining an inside chamber for storing a gas to be measured.
The measured gas side cover 2′ has a nested structure composed of a plurality cup-shaped cover members 21′, 22′ disposed one inside another. Each of the cover members 21′, 22′ has a gas inlet hole 210′, 220′ formed in a side wall thereof for introducing the gas to be measured, and a bottom hole 219′, 229′ formed in a bottom wall thereof.
An air-fuel ratio sensor element used for the air-fuel ratio sensor has the same structure as widely known sensor elements used for detecting an oxygen concentration. The air-fuel ratio sensor element is comprised of a solid electrolyte of plate-like or cup-shaped configuration and a pair of electrodes provided on surfaces of the solid electrolytic body. One electrode that is designed for contact with the measured gas has on its surface a diffusion resistance layer. Typical examples of such air-fuel ratio sensor element are disclosed in Japanese Patent Laid-open Publications (JP-A) Nos. 10-123089 and 2001-108650.
Apart from applications in which the air-fuel ratio sensor is used in an atmosphere of burnt gas, such as exhaust gas from an internal combustion engine that is substantially free from flammable or combustible gas especially fuel vapor and the like, an application in which the air-fuel ratio sensor is used in an atmosphere of unburnt gas, such as fuel vapor containing a great quantity of combustible gas, may encounter a problem as discussed below.
When the air-fuel ratio sensor is exposed to an unburnt gas atmosphere that contains large amounts of fuel vapor such as butane, reaction between butane and oxygen on a surface of the sensor element is insufficient so that oxygen to be reacted remains un-reacted and is allowed to reach the electrode. Furthermore, because oxygen has a higher diffusivity than the fuel vapor such as butane with respect to the diffusion resistance layer, the vicinity of the electrode of the air-fuel ratio sensor apparently becomes oxygen-enriched as compared to oxygen concentrations achieved when unburnt gas and oxygen in the measured gas react fully. Consequently, the output from the air-fuel ratio sensor may cause a lean shift, failing to achieve a correct air-fuel ratio measurement.
With the foregoing problem associated with the prior art devices in view, an object of the present invention is to provide an air-fuel ratio sensor which is capable of measuring an air-fuel ratio in an unburnt gas atmosphere with accuracy.